How can error messages be effectively implemented in PHP to inform users of restricted actions in a shopping cart?

To effectively inform users of restricted actions in a shopping cart in PHP, error messages can be implemented using conditional statements to check for restricted actions such as adding an out-of-stock item or exceeding the maximum quantity allowed. When a restricted action is detected, an error message can be displayed to the user to inform them of the issue.

// Check if the user is trying to add an out-of-stock item to the shopping cart
if($item['stock'] <= 0){
    echo "Error: This item is currently out of stock.";
}

// Check if the user is trying to exceed the maximum quantity allowed for an item
if($quantity > $max_quantity){
    echo "Error: You can only purchase a maximum of ".$max_quantity." of this item.";
}